In a vehicle, a damper is usually arranged in a torque transmission path to absorb torque vibration, which is generated at a power source such as an engine or an electric motor. When a large torque is generated at the power source, such a damper blocks the transmission of torque from one side of the torque transmission path to the other side of the torque transmission path with a limiter mechanism (see Japanese Laid-Open Patent Publication No. 2008-274968).
Japanese Laid-Open Patent Publication No. 2008-274968 describes a damper (hereinafter referred to as “conventional damper”), which is of a wet-type damper used in a so-called hybrid vehicle, which uses an engine and an electric motor as a power source. As shown in FIG. 13, the conventional damper includes an outer case 303, which is formed by a front cover 301 serving as a first cover and a rear cover 302 serving as a second cover. The front cover 301, which is generally cylindrical and has a closed bottom, is connected to an output shaft of the engine. The rear cover 302, which is generally annular when viewed from the front, is welded and fixed to the circumference of the front cover 301. The outer case 303 is filled with lubricant oil serving as a liquid. A sleeve 304 projects rearward from a radially inner portion of the rear cover 302. A planetary gear mechanism (not shown) has an input shaft (output member) 305, the front end of which is inserted into the outer case 303.
The outer case 303 houses a generally cylindrical hub 306, a damper device 307, and a limiter mechanism 308. The hub 306 is supported to be integrally rotatable with the input shaft 305 of the planetary gear mechanism. The damper device 307 is arranged radially outward from the hub 306. The limiter mechanism 308 is arranged radially outward from the damper device 307. The damper device 307 includes an annular central plate 309, to which the torque from the power source is transmitted through the limiter mechanism 308, and an annular intermediate member 310, which is supported to be integrally rotatable with by the hub 306. A torque absorber 312 is arranged in the torque transmission path between the central plate 309 and the intermediate member 310. The torque absorber 312 has a damper spring 311 serving as an elastic member that is elastic in the circumferential direction.
The limiter mechanism 308 includes an annular disc spring (limiter biasing member) 313 and a limiter plate 314. The disc spring 313 is supported by a radially outer portion of the rear cover 302. The limiter plate 314 is arranged between the disc spring 313 and a radially outer portion of the central plate 309. The disc spring 313 applies biasing force to the limiter plate 314, which further applies to biasing force to the central plate 309. At the radially outer portion of the central plate 309, friction members 315 and 316 are respectively arranged on the surface of the central plate 309 facing toward the limiter plate 314, and a front surface of the central plate 309 facing toward the radially outer portion of the bottom of the front cover 301. The limiter plate 314 pushes the central plate 309 via the friction member 315 with the biasing force of the disc spring 313. The central plate 309 further pushes the bottom of the front cover 301 via the friction member 316. Thus, when the outer case 303 rotates, frictional force is generated between the friction member 315 and the limiter plate 314 and between the friction member 316 and the front cover 301. This rotates the central plate 309 with the outer case 303.
If the torque from the power source does not reach a predetermined torque, the torque from the power source is transmitted via the outer case 303, the limiter mechanism 308, the damper device 307, and the hub 306 to the input shaft 305 of the planetary gear mechanism. If an excessively large torque is generated and the torque exceeds a predetermined torque when the vehicle is driven by the engine or when starting the engine with the electric motor, slipping occurs between the limiter mechanism 308 and the central plate 309 of the damper device 307. In other words, if torque that is greater than the frictional force generated between the limiter mechanism 308 and the damper device 307 is input to the conventional damper, the transmission of an excessively large torque between the limiter mechanism 308 and the damper device 307 is blocked.
When assembling the outer case 303 in the conventional damper, the front cover 301 is welded to the rear cover 302 in a state in which the damper device 307 and the limiter mechanism 308 are accommodated in the rear cover 302. In this state, the frontward biasing force of the disc spring 313 in the limiter mechanism 308 is applied via the limiter plate 314 and the central plate 309 to the front cover 301. Thus, the radially outer portion of the front cover 301 must be welded with the radially outer portion of the rear cover 302 while pressing the front cover 301 against the rear cover 302 to maintain the predetermined positional relationship of the covers 301 and 302 in the axial direction. Since pressing force must be applied to the front cover 301 or the rear cover 302 when coupling the covers 301 and 302, the assembling of the outer case 303 is extremely difficult.
Furthermore, the front cover 301 may be deformed when pressed against the rear cover 302 when assembling the outer case 303.
Moreover, the friction members 315 and 316 have friction coefficients (μ) that generally vary greatly. Thus, the limiter mechanism 308 may not function even if the predetermined torque cannot be reached. In particular, with a dry-type friction member, the friction coefficient (μ) increases when its engagement surface becomes rusted. This may overly increase the torque (critical torque) at which torque transmission is blocked. As a result, excessive torque may inflict damages to the damper and other mechanisms in the torque transmission path.
Accordingly, in the prior art, each mechanism arranged in a torque transmission path must be designed taking into consideration the variations in critical torque. This inevitably enlarges each mechanism. For example, the outer diameter of the input shaft 305 in the planetary gear mechanism must be increased, and the dimensions of the damper spring 311 must be increased.
Additionally, an impact torque is alleviated and absorbed as the damper spring 311 compresses and deforms. Thus, the damper spring 311 has a relatively low spring constant so as to be suitable for alleviating such impact torque. In contrast, however, a large impact torque may not be sufficiently alleviated if the spring constant is lowered. Further, the energy of the impact torque cannot be readily absorbed just with the damper spring 311.
It is an object of the present invention to provide a damper that allows for an outer case to be easily assembled without being deformed. It is another object of the present invention to provide a damper that suppresses variations in the friction coefficient of a friction member to stabilize the critical torque, while smoothly alleviating a large impact torque.